The present invention relates to a method of producing an integral self supporting coating test piece from a coating material.
Coating materials are used to cover the surfaces of metallic articles, or components, so as to protect the metallic articles from degradation when subjected to hostile working conditions. An example of such a metallic article is a turbine blade, or a turbine vane, which operates in a gas turbine engine.
It is known to make turbine blades, or turbine vanes, from a given material and then coat the aerofoil portions of the turbine blades, or turbine vanes, with another material more able to withstand the high temperature gases that flow over the surfaces of the aerofoil portions of the turbine blades, or turbine vanes. The coating may be a metal alloy layer, a ceramic layer, a mixture of metal and ceramic layer or a combination of a metal layer and a ceramic layer. The coating is normally applied by the known techniques of thermal spraying and plasma spraying and the coating is very thin. It is important to have some idea of the structural integrity of the coating itself, and the quality of its adhesion to the aerofoil portions of the turbine blades, or turbine vanes, prior to actual use of the coating on the turbine blades or turbine vanes.
There are a number of difficulties when trying to obtain data from coatings. Firstly coatings are generally too thin to gain useful results when the coating is attached to an article. Secondly the article is often stronger and/or stiffer than the coating and the effects of the article contaminate the test results. Thirdly the coating produced by thermal spraying, or plasma spraying, has a particulate structure with different properties to cast materials of the same composition, thus it is not possible to look up material data from a data book of predetermined compositions. Fourthly the coating itself may be a composite material with complex properties, e.g. the coating is a mixture of metal and ceramic.
When making a coating test piece, it is known to provide a salt based flat slab and to thermally spray a particulate coating material on to one surface of the salt slab. During the thermal spraying, or plasma spraying, of particulate material, on contact with a surface of an article the molten particles cool rapidly and contract. Molten particles following and contacting the semi-cooled particles generate residual stresses in the coating material. If as sometimes happens, the bond between the coating and the salt slab is not sufficiently strong, the coating delaminates. Further if the salt slab is not sufficiently stiff, again as sometimes happens, the aforementioned residual stresses may induce distortion into the coating material and the salt slab. The resulting poor quality coating, when removed from the salt slab has too frequently proved unable to maintain an integral self supporting form that would give sufficiently accurate indication of strength when stressed on a standard mechanical test machine.
It is also known, from our published European patent application EP1600522A2, to provide an integral self supporting coating test piece from particulate material by providing a dissolvable mould, made from a dissolvable plastic, thermally spraying particulate material onto the dissolvable mould and then dissolving the dissolvable mould away from the coating test piece. However, there is a problem with this method of producing a coating test piece in that a new dissolvable mould has to be made for each coating test piece produced. The dissolvable moulds provide limited mechanical stability, which may lead to high scrap rates.